The present invention relates to an optical storage medium in or from which data is recorded or reproduced with laser radiation. Particularly, this invention relates to an unrewritable dual-layer optical storage medium having two organic-dye recording layers.
Optical storage media having organic-dye recording layers recently available are, for example, writable compact discs (CD-R) and writable digital versatile disks (DVD-R) for archival recording of music or video data.
DVD-Rs having large storage capacity are especially used for recording video data. There is, however, a demand for larger storage capacity for long-time recording.
One way to achieve larger storage capacity is to make smaller a laser spot so that it can carry highly dense data when a laser beam is radiated onto a recording layer of an optical storage medium from a laser source of a recording/reproducing apparatus.
Possible ways to have a smaller laser spot use a shorter laser wavelength in the range of blue, higher Numerical Aperture (NA) for an objective lens of an optical pickup installed in a recording/reproducing apparatus, etc.
These ways are, however, not feasible in reproduction compatibility with existing DVD recording/reproducing (or reproduction-dedicated) apparatus.
One recently proposed medium structure that achieves larger storage capacity with better reproduction compatibility is a multi-layer, such as, a dual-layer structure in which an optical storage medium have two or more recording layers in laminated layers.
For example, Japanese Patent Publication No. H08 (1996)-23941 discloses a dual-layer optical storage medium in which a reflective layer, a cured resin (as a recording surface), etc., are laminated on one substrate.
Japanese Patent Unexamined Publication No. H10 (1998)-283682 discloses a method of producing a dual-layer optical storage medium having two substrates. Each substrate is made of a transparent resin and has grooves formed thereon. Formed on each substrate by specific procedures are a recording layer and other layers. The substrates are bonded to each other with an ultra-violet cured resin so that the layers are sandwiched therebetween.
Each substrate can be produced through well-known injection molding, like a substrate having a single recording layer. Application of a dye, formation of a reflective film on each substrate can also be done in the same way as or a similar way to such a substrate having a single recording layer.
This type of bonding structure can be achieved with high productivity through well known simple techniques.
FIG. 1 shows a dual-layer optical storage medium D2 having such a bonding structure.
The optical storage medium D2 has a first laminated layer structure L0 and a second laminated layer structure L1.
The laminated layer structure L0 consists of a transparent substrate 21, a recording layer 22, and a semi-transparent film 23. One surface of the substrate 21 is a light-incident plane 21a via which a laser beam is incident in recording or reproduction. Spiral or concentric grooves 21G and lands 21L are alternately formed on the other surface of the substrate 21. The grooves 21G and lands 21L are coated with an organic resin (the material of the recording layer 22). Formed on the layer 22 is the semi-transparent film 23 made of a metal or an alloy.
The laminated layer structure L1 consists of a substrate 27, a reflective film 26, and a recording layer 25. Spiral or concentric grooves 27G and lands 27L are alternately formed on the substrate 27. The grooves 27G and lands 27L are covered with the reflective film 26. Applied on the film 26 is an organic resin (the material of the recording layer 25).
The semi-transparent film 23 of the laminated layer structure L0 and the recording layer 25 of the laminated layer structure L1 are bonded to each other via a bonding layer 24.
Further layers may be provided in the laminated layer structure L0 and/or the laminated layer structure L1 for higher performance, reliability, etc.
In recording or reproduction, laser beams LA1 and LA2 are incident on the layers L0 and L1, respectively, via the light-incident plane 21a. 
Illustrated in FIG. 1 is a known recording mechanism that is called on-in recording in which data is recorded in the laminated layer structure L0 in on-groove recording whereas another data is recorded in the laminated layer structure L1 in in-groove recording.
In general, data is recorded on convex sections of a substrate, that stick out toward a light-incident plane in on-groove recording. In contrast, data is recorded on concave sections of the substrate, that cave in when viewed from the light-incident plane in in-groove recording.
As illustrated with dot lines in FIG. 1, in the on-groove recording, the laser beam LA1 is incident on the laminated layer structure L0 via the light-incident plane 21a so that data is recorded on the recording layer 22 formed on the grooves 21G. The grooves 21G formed on the substrate 21 are convex sections that stick out toward the plane 21a. 
In contrast, in the in-groove recording, the laser beam LA2 is incident on the laminated layer structure L1 via the light-incident plane 21a so that data is recorded on the recording layer 25 formed on the grooves 27G. The grooves 27G formed on the substrate 27 are concave sections that cave in when viewed from the plane 21a. 
It is, however, practically difficult to achieve the on-in recording. The following on-on recording is more practical and used often. In this recording mechanism, data are recorded in the laminated layer structures L0 and L1 both in on-groove recording. In detail, recording to the structure L0 is the same as in the on-in recording. In addition, another data is recorded in the structure L1 in such a way that it is recorded on convex sections of a substrate, that stick out toward a light-incident plane. These convex sections correspond to the lands 27L, formed on the substrate 27 (FIG. 1), that stick out toward the light-incident plane 21a, although on-groove recording to the structure L1 is not illustrated in FIG. 1.
A dual-layer optical storage medium having a bonding structure, such as, shown in FIG. 1 and disclosed in Japanese Patent Unexamined Publication No. H10(1998)-283682 is better in productivity than that disclosed in Japanese Patent Publication No. H08(1996)-23941.
On-groove recording to the laminated layer structure L1, in such a bonding structure shown in FIG. 1, however, has the following disadvantages:
In FIG. 1, when the recording layer 25 is formed by spin coating, the grooves 27a are well filled with an organic dye, the material of the layer 25, whereas it is difficult to apply the organic dye on the lands 27L with enough and uniform thickness.
This results in that on-groove recording to the laminated layer structure L1 in an optical storage medium having the bonding structure, such as shown in FIG. 1, cannot provide feasible reproduction output characteristics.
Nevertheless, in-groove recording to the laminated layer structure L1 in the on-in recording mechanism, however, has the following disadvantages:
Recording to both of the laminated layer structures L0 and L1 is conducted by denaturalizing the organic dyes of the recording layers 22 and 25.
In recording to the laminated layer structures L0 and L1 with the same laser power, the organic dye of the recording layer 22 is denaturalized enough for superior recording quality whereas that of the recording layer 25 is not denaturalized enough, thus causing inferior recording quality.
Higher laser power to the recording layer 25 causes not only denaturalization of the organic dye of the recording layer 25 but also deformation of the substrate 27 because high power laser is focused on the center section of each groove 27G of the laminated layer structure L1.
The following is a possible reason of such unfeasible phenomenon:
In the optical storage medium D2 having the bonding structure, shown in FIG. 1, the semi-transparent film 23 is formed substantially flat on the recording layer 22 in the laminate layer structure L0. Contrary to this, in the laminated layer structure L1, the reflective film 26 is formed on the substrate 27 before the recording layer 25 is formed so that the film 26 has concave and convex sections like the grooves 27G and the lands 27L.
In other words, the organic dye of the recording layer 22 in each groove 21G is contact with the semi-transparent film 23 only at the upper surface of the dye in the laminated layer structure L0. On the contrary, in the laminated layer structure L1, the organic dye of the recording layer 25 in each groove 27G is contact with the reflective film 26 at side walls and the lower surface of the dye.
The reflective film 26 exhibits high thermal conductivity because it is made of a metal, an alloy or a metal compound.
Such thermal property could cause dissipation of heat carried by a laser beam to the recording layer 25 through the reflective film 26 contacted with the groove side walls. Unfeasible heat dissipation would not allow enough thermal action to the organic dye of the recording layer 25, which results in insufficient denaturalization of the dye, thus not giving high modulation rate.